To provide sounds of higher quality with television receivers, attempts have been made in recent years to simulate the perception at a live performance by placing emphasis on the reproduction of low sounds which has not been considered very important.
Unexamined Japanese Utility Model Publication SHO 58-50781 discloses a speaker system for low sounds for use in television receivers. With reference to FIG. 5 showing the disclosed system, a speaker cabinet 3 provided at the lower portion of a TV cabinet 1 houses speakers 12 for medium to high sounds, drive speakers 14 for low sounds and horns 16.
However, this speaker system has the problem of necessitating a large speaker cabinet to increase the overall volume of the television receiver since the horns need to have an increased volume for the reproduction of low sounds, i.e., sounds of low frequencies.
On the other hand, speaker systems of the so-called Kelton type are known as speaker systems having a small speaker cabinet and nevertheless adapted for the reproduction of low sounds.
FIG. 4 is a diagram showing the principle of the speaker system. The speaker cabinet 3 comprises a first chamber 22 surrounding the rear side of a speaker 5 and formed with a duct 20, and a second chamber 24 surrounding the front side of the speaker 5.
The first chamber 22 is formed based on the principle of Helmholtz resonator. The sound waves radiated from the rear side of the speaker 5 resonate at a specific frequency (termed the "cabinet resonance frequency") according to the correlation between the inside volume of the first chamber 22 and the mass of the air in the duct portion 20, and are released from the cabinet 3.
The cabinet resonance frequency is expressed by the following equation. ##EQU1## wherein fr: cabinet resonance frequency (Hz)
C: speed of sound (m/s) PA0 S: cross sectional area of the duct (m.sup.2) PA0 L: length of the duct (m) PA0 V: volume of the first chamber (m.sup.3)
The cabinet resonance frequency fr needs to be lower in order to extend the range of reproduction of low frequencies for the reproduction of low sounds. The cabinet resonance frequency fr can be decreased by increasing the volume V of the first chamber, but this results in the problem of making the speaker cabinet larger. Accordingly, the frequency fr is decreased for the reproduction of low sounds by decreasing the volume V of the first chamber and suitably increasing the length of the duct 20.
The second chamber 24 confines the sound waves radiated from the front side of the speaker 5, inhibiting the vibration of the cone type diaphragm of the speaker S owing to the acoustic stiffness of air.
The second chamber 24 is provided for the following reason. According to the principle of Helmholtz resonator, the resonance frequency at the duct portion 20 of the first chamber 22, i.e., the cabinet resonance frequency, is a frequency when the amplitude of the speaker diaphragm becomes very small. However, although the amplitude of the diaphragm is small at the cabinet resonance frequency, the amplitude increases at frequencies different from the cabinet resonance frequency. The amplitude then gives undesired vibration to the diaphragm to flutter the diaphragm and break the diaphragm.
The degree to which the vibration of the diaphragm is inhibited is dependent on the volume of the second chamber. If the volume is small, the acoustic stiffness of air is great, consequently inhibiting the vibration of the diaphragm greatly and rendering the diaphragm less likely to vibrate. Nevertheless, this means an increase in the lowest resonance frequency of the speaker to restrict the reproduction range of low sounds. In order to reproduce sounds of low frequencies, for example, of up to about 45 Hz, it is therefore necessary to increase the weight of the diaphragm of the drive coil, consequently leading to the problem of a reduced acoustic conversion efficiency.
If the volume of the second chamber is great, on the other hand, the vibration of the diaphragm will not be inhibited effectively since the stiffness of air is then small. Accordingly, if the second chamber has too great a volume, the provision of the second chamber becomes meaningless, and the diaphragm will flutter and become broken. Thus, there is a limitation to the increase in the volume of the second chamber.
Because of the problems described above, the Kelton-type speaker has found no use in television receivers. For use in television receivers, therefore, it has been desired to provide a compact speaker system which is adapted to efficiently reproduce sounds of low frequencies of up to about 45 Hz.
We have developed a speaker system which is compact and yet capable of efficiently reproducing sounds of low frequencies of up to about 45 Hz based on the finding that the same effect as an increase in the volume of the second chamber can be achieved when the second chamber is formed with apertures with an opening degree in a predetermined range.